When using a custom block or item with a vanilla trigger (e.g. inventory changed), it's automatically set to "air" in the advancement .json
I haven't checked if this issue is there with 1.14 generators.
Issue comments
We have released a patch update to fix this and a few other bugs:
2020.1.05113
- [Bugfix, 1.14.4] Custom dimension portals did not work
- [Bugfix, 1.14.4] Data pack pack_format was set to 1 instead of 4
- [Bugfix #56503] Vanilla advancement triggers did not work with custom blocks and items
- [Bugfix #56505, 1.14.4] Advancement name and descriptions did not work in data packs
- [Bugfix #56537, 1.14.4] Comparator item was mapped to a composter
If you have update notifications turned on in MCreator, you will be notified if you need to patch on your next launch of MCreator.
To apply patch, redownload and reinstall MCreator 2020.1
I can confirm this bug. Too bad you did not report this when snapshots were there, I will release a patch if other problematic bugs will get found, otherwise this one will wait for 2020.2. Turn on the update notifications if there will be a patch ;)